A deep drawing steel sheet is conventionally used for forming in which the height of press forming is high, or the shape is complicated, for example, forming automobile components such as an oil pan, etc. As a mechanical property required for this deep drawing steel sheet, it is necessary that the r value (Lankford value, abbreviated to "the r value" hereinafter), particularly the average r value ((r.sub.L +2r.sub.D +r.sub.C)/4 wherein r.sub.L, r.sub.D and r.sub.C indicate r values in the rolling direction, the direction at 45.degree. with the rolling direction and the direction perpendicular to the rolling direction, respectively), is high. It has been considered that when the planar anisotropy of r values .DELTA.r=(r.sub.L +r.sub.C)/2-r.sub.D is low, uniform drawing is possible with high yield. It has also been considered that an effective manner of increasing the r value was to decrease .DELTA.r.
Therefore, conventional development of materials has progressed from this viewpoint, and a lot of effort has been made for this purpose. For example, a cold-rolled steel sheet comprising extra low-C steel (C.ltoreq.0.008 wt %) to which a carbide forming element such as Ti, Nb or the like is added has been developed. Further, the technique of obtaining a higher r value, e.g., an average r value of 2.6 or more, by warm lubrication rolling of the extra low-C steel has recently been proposed in, for example, Japanese Patent Unexamined Publication Nos. 64-28325 and 2-47222.
However, even for such a steel sheet having an ultra-high r value, actual rectangular drawing sometimes causes breakage during press forming. "Rectangular drawing" means such asymmetrical drawing deformation as shown in FIG. 3, unlike axially symmetric cupping. In order to avoid such breakage, an attempt has been conventionally made to simply increase the average r value or decrease .DELTA.r on the basis of the thinking that the breakage is due to an insufficient r value, and a lot of effort has been made to further improve the steel sheet production process. However, the breakage cannot be effectively prevented yet.
In detailed examination of such breakage portions, not only .alpha. breakage (breakage from a punch shoulder), which is often observed in a normal deep drawability test (cup forming), but also wall breakage, i.e., breakage from an intermediate position of the corner wall, often occur. Such types of breakage do not occur as often in cupping, and can be said to be peculiar to rectangular forming.
There are few researches on wall breakage in rectangular forming, and it is known from, for example, "Plasticity and Working", Vol. 10, No. 101 (1969-6), P. 425, that the occurrence of wall breakage tends to be suppressed by increasing strength and T value (thickness strain at the time of occurrence of breakage in pure bulging), or decreasing the crystal grain diameter.
However, components such as an oil pan and the like which have a high height of forming are required to have high average r values, and thus have a problem in that it is difficult from the viewpoint of mechanical properties to satisfy a high r value, and high strength and a fine grain diameter, which cause a decrease in the r value. With respect to the T value, there is a problem in that no effective means for increasing the T value is known.
As described above, the fact is that since there are few researches on mechanical properties in such forming as rectangular forming, what factors of a steel sheet affect the wall breakage which occur in rectangular forming have been hardly known yet. Under these conditions, in fact, a steel sheet having mechanical properties suitable for rectangular forming or a production method thereof are hardly investigated.